The invention relates to an adhesive bond between a substrate material whose surface and solid region proximate to the surface have polymer compounds with a low active surface energy, and another material, as well as a method for producing a corresponding adhesive bond. In particular, the invention relates to a metallized adhesive fluoropolymer, such as polytetrafluoroethylene (PTFE) as a base material (substrate material) for printed circuit boards with a high structural density (finely and very finely patterned printed circuit boards) for applications in the GHz frequency range and to a method for adhesive metallization of a corresponding fluoropolymer.
An adhesive bond between two different materials, for example between a substrate material and another material, is absolutely necessary for a large number of technical applications of corresponding adhesive bonding materials. For example, the adhesive metallization of surfaces of a polymer material with extremely favorable dielectric properties (small dielectric constant ∈rel and small dielectric loss tan δ) are an essential prerequisite for the production of high-quality printed circuit boards with a high structural density operating at frequencies above 1 GHz. To minimize electrical losses, in particular on printed circuit boards with very narrow conductor tracks, the surfaces of the metallic conductor tracks must be very smooth. This means that the adhesive bond between the fluoropolymer as the substrate material and the metallic conductor tracks must be implemented without severely roughening the substrate material. Similar requirements apply to materials for the production of low loss electric capacitors.
GB 816641 discloses a method for metallizing a PTFE surface, whereby the PTFE surface is initially treated with sodium dissolved in liquid ammonia, followed by the deposition on the PTFE surface of a nickel layer produced from a solution of a nickel salt and sodium hypophosphite. The nickel layer forms the base for the deposition of another metal layer.
DE 198 17 388 A1 also describes a solution where the smooth surface of a fluoropolymer is initially cleaned and etched by a corona discharge process, with the process being performed at an operating pressure in a range of 10 Pa (10−1 mbar) to 1,500 Pa (15 mbar). The pre-treatment conditions are adjusted so as to produce a very smooth substrate surface. It has been observed with this type of corona discharge that a very smooth surface can be formed with an oxygen/tetrafluoromethane-mixture after the initial etch step. A first metal layer containing nickel is deposited on this activated surface by decomposition of volatile nickel compounds, and a second metal layer is subsequently deposited on the nickel layer from a metallizing bath. According to statements by the applicant, a polymer/metal composite formed in this manner has a surprisingly high bond strength. Both solutions have the disadvantage that essentially a nickel layer has to be deposited first on the surface of the fluoropolymer. The adhesive strength of the produced composite is also insufficient for many technical applications.
A similar approach is disclosed in DE 101 63 437 A1. However, the cleaning step and the initial etch step are performed in a vacuum chamber at an operating pressure of 0.6 Pa (6×10−3 mbar). The surface is likewise coated with carbon at 0.6 Pa, whereby the coating is performed by HF cathode sputtering. The produced sandwich construction is subsequently processed in several processing steps and is then adhesively bonded to a metal. It is stated in DE 101 63 437 A1 that the adhesive bond produced in this manner could not be broken. Disadvantageously, however, an additional material has to be deposited first. It should also be noted that the high bond strength claimed in the application could generally not be confirmed.
U.S. Pat. No. 6,342,307 B1 also describes a method for producing an adhesive bond between a metal layer and a polymer surface. This method includes the following substantial process steps:
1. A metal is deposited on the surface of the polymer without forming a continuous metal layer; instead, deposits in the form of metal particles with dimensions in the range from 5 to 20 nm are formed. The processing conditions are selected so as to have no effect on the polymer surface (temperature below the glass transition temperature).
2. The polymer surface is then heated to a temperature above the glass transition temperature, and the particles are incorporated into the surface layer of the polymer so as to be embedded in the polymer at least halfway, but not completely. After a cool-down, the particles protruding from the polymer surface are firmly anchored in the surface.
3. Thereafter, metal is again deposited onto the surface prepared in this manner, whereby the process is carried out so as to form a continuous metal layer. The added metal bonds with the metal particles protruding from the surface of the polymer, whereby the particles which are firmly embedded in the polymer produce a metal layer which is firmly anchored on the polymer surface.
Care has to be taken in all process steps that the metal is deposited without undergoing a chemical change. In particular, care has to be taken that the metal is not oxidized. In the process described in U.S. Pat. No. 6,342,307 B1, a positively locked bond is formed between the metal and the polymer in addition to the adhesion forces. Disadvantageously, embedding metal particles in the polymer causes the side of the metal layer facing the polymer to be highly structured. It is also known to expose bumpers made from polymers to an air plasma before applying paint to produce a durable paint coat, whereby the coating becomes particularly durable due to the increased surface energy of the plastic (Herold, Dr., Martin, Modifikation von Festkörperoberflächen und ihre Charakterisierung durch Ellipsometrie, Dissertation Universität Tübingen, 2001—Herold, Dr., Martin, Modification of solid surfaces and their characterization by ellipsometry, Dissertation University Tübingen, 2001). However, the described solution is capable of producing an adhesive bond in only limited applications.
The invention addresses the problem of providing an adhesive bond between a substrate material, whose surface and solid region proximate to the surface include polymer compounds with a low active surface energy, and another material, as well as providing a method for producing a corresponding adhesive bond, so as to overcome the disadvantages of the present state-of-the-art.